COSMOS-DASH: The Evolution of the Galaxy Size-Mass Relation since z ∼ 3 from New Wide-field WFC3 Imaging Combined with CANDELS/3D-HST

We present COSMOS-Drift And SHift (DASH), a Hubble Space Telescope (HST) Wide Field Camera 3 (WFC3) imaging survey of the COSMOS field in the H ₁₆₀ filter. The survey comprises 456 individual WFC3 pointings corresponding to an area of 0.49 deg² (0.66 deg² when including archival data) and reaches a 5σ point-source limit of H ₁₆₀ = 25.1 (0.″3 aperture). COSMOS-DASH is the widest HST/WFC3 imaging survey in the H ₁₆₀ filter, tripling the extragalactic survey area in the near-infrared at HST resolution. We make the reduced H ₁₆₀ mosaic available to the community. We use this data set to measure the sizes of 169 galaxies with at 1.5 < z < 3.0 and augment this sample with 749 galaxies at 0.1 < z < 1.5 using archival ACS imaging. We find that the median size of galaxies in this mass range changes with redshift as r_eff = (13.4 ± 0.5) × (1+z)^{(-0.95 ± 0.05} kpc. Separating the galaxies into star-forming and quiescent galaxies using their rest-frame U - V and V - J colors, we find no statistical difference between the median sizes of the most massive star-forming and quiescent galaxies at : they are 4.9 ± 0.9 kpc and 4.3 ± 0.3 kpc, respectively. However, we do find a significant difference in the Sèrsic index between the two samples, such that massive quiescent galaxies have higher central densities than star-forming galaxies. We extend the size-mass analysis to lower masses by combining it with the 3D-HST/CANDELS sample of van der Wel et al. and derive empirical relations between size, mass, and redshift. Fitting a relation of the form r_eff = A × m_∗ ^α, with m_∗ = M_∗/5 × 10¹⁰ M_⊙ and r _eff in kpc, we find log A = -0.25 log(1 + z) + 0.80 and α = -0.13 log(1 + z) + 0.27. We also provide relations for the subsamples of star-forming and quiescent galaxies. Our results confirm previous studies that were based on smaller samples or ground-based imaging.